The reaction of blast furnace slag in sodium hydroxide (NaOH) solutions of different molarities is evaluated. The compressive strength of the activated slag does not scale with the molarity of NaOH. The primary reaction product in the activated slag is identified with calcium aluminosilicate hydrate [C(A)SH]. While the early reactivity of slag is enhanced at higher alkalinity, and the dissolution of slag increases with the molarity of NaOH, the quantity of C(A)SH in the hydrating system does not scale with the molarity of NaOH in the activated slag. From X-ray diffraction (XRD) analysis, an additional water-soluble, sodium-based amorphous product is identified in the reaction products. The water-soluble product, which does not contribute to strength, increases proportionately with the initial Na content in the solution. At higher molarity, there is a larger proportion of the water-soluble product relative to C(A)SH in the reaction products. The Ca/Si ratio and Al/Si ratios in the C(A)SH gel are relatively invariant of the NaOH molarity in the activating solution. The compressive strength gain in the alkali-activated slag is determined by the quantities of C(A)SH and the intrinsic sodium-filled water-soluble product. © 2020 American Society of Civil Engineers.